Update gem dependencies (Allow Rails 3.2)

Also set rspec to development_dependency, and fixed tests to run on
Rails 4
Use URI to build HTTP request instead of String

Much cleaner code. Credits to @dkalachov
Add dlr-mask config parameter

Also removed dlr-url configuration. This can be passed as an option on a
per request basis as is its usual use case.
Add test for passing extra parameters to kannel
Update README to reflect changes
Bump version
Merge branch 'refactor_http_code'

Custom params now allowed. Some breaking changes to config