Add descriptions for proposal fields

WTForms offers a `description` argument to give more information about
form fields. These descriptions will hopefully give users a better
understanding of the purpose of each field.

Styles are being included to give a better visual appearance to these
descriptions.

Some of this text is taken from the PyCon website.

Closes #27
Merge pull request #30 from PyGotham/cfp-descriptions

Add descriptions for proposal fields