Add support for application/x-www-form-urlencoded

[tvh]

This depends on #9. It support for url encoded request bodys. To use a custom type, There is these typeclasses:

class ToFormUrlEncoded a where
  toFormUrlEncoded :: a -> [(TextL.Text, TextL.Text)]

class FromFormUrlEncoded a where
  fromFormUrlEncoded :: [(TextL.Text, TextL.Text)] -> Either String a

[coveralls]

Coverage decreased (-4.99%) to 53.59% when pulling bda1c30 on anchor:form-urlencoded into fb71eb8 on haskell-servant:master.

[coveralls]

Coverage increased (+30.3%) to 88.89% when pulling 937e7c2 on anchor:form-urlencoded into fb71eb8 on haskell-servant:master.

[christian-marie]

I'm going to wait till the dependent PR is merged to review

[alpmestan]

Looks good, once #9 is merged in. We'll just have to rebase this PR to make sure travis gives us the go-ahead. Cheers!

[jkarni]

Good stuff. Mind adding a few more tests? Since the encoding/decoding is in-house, just checking inverse laws doesn't quite seem like enough.

[coveralls]

Changes Unknown when pulling 6ef48cf on anchor:form-urlencoded into * on haskell-servant:master*.

[coveralls]

Changes Unknown when pulling 1d378e6 on anchor:form-urlencoded into * on haskell-servant:master*.

[tvh]

I just added 2 more roundtrip-tests using a different library..

The reason I don't use that library in the first place is

1. Avoid lots of string-conversions
2. Avoid extra import for very simple functionality

[coveralls]

Changes Unknown when pulling 4f91a28 on anchor:form-urlencoded into * on haskell-servant:master*.

[jkarni]

Thanks!