Parse IP addresses, MAC addresses, and email addresses

add support for pointers and TextUnmarshaler

Add support for time.Duration fields

Add support for environment variables

Resolve some merge conflicts from #8

As with the previous key added in 19c0916, I deleted the associated
private key using /usr/bin/shred.  But of course, I have no way to prove
that.  Again, all the more reason to always use a test account when
running these integration tests.

see 92bbb1ce51

This adds support and integration tests for editing branch protection.

MustParse returns *Parser

For more information about the preview see here:
https://developer.github.com/changes/2015-11-11-protected-branches-api/

This adds only retrieving the information and not setting it. Setting
will come in a later commit (if that one here is okay).

I tried to follow the naming guidelines as much as possible and
hopefully I made it right the first time. :-)

Adding the new BasicAuthTransport raises the minimum required version of
go to 1.4.

Add a new error type, TwoFactorAuthError, which identifies the need to
include a one-time password when using Basic Auth for a user with
two-factor auth enabled.

An example of using both the new transport and error type can be seen in
examples/basicauth/main.go.

fixes #258

Both:
https://developer.github.com/v3/issues/#list-issues
https://developer.github.com/v3/issues/#list-issues-for-a-repository

State that the default is `desc` for direction. (As does my results). So
update the comment.

In particular, this handles the case of trying to decode No Content
(204) responses.

Previously, concurrent calls to Client.Do would have data races writing
Client.Rate. Protect it with a mutex.

Resolves #253.

Fix error when printing usage for multi-value arguments

We try to compare []strings, which are uncomparable types:

`panic: runtime error: comparing uncomparable type []string`

Add default values to usage

The value "all" is also possible for State. See https://developer.github.com/v3/issues/#list-issues
and https://developer.github.com/v3/issues/#list-issues-for-a-repository.

Check if the value isn't it's zero value and if not add a default value
to the usage text.

Add built ins to options in help output