add support for pointers and TextUnmarshaler

Add support for time.Duration fields

Add support for environment variables

Resolve some merge conflicts from #8

MustParse returns *Parser

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

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

Adds help to the options in help output with an easy way to add more
built ins.

Move package documentation to doc.go

Fix bug with error not being written to stderr

Fix lint warning

Only the usage message was written to stderr, the error was written with
the standard fmt.Println.

Write usage message to stderr on error

Remove excess trailing whitespace from Usage generation

When the parsing of parameters/flags fails eg. when a required flag is
missing, print the usage statement and error to stderr instead of
stdout.

Add coveralls badge

Add more tests for parser

add tests for usage info

add coveralls to .travis.yml