%Maxwell{} => %Maxwell.Conn{} and move help function to conn.ex

[zhongwencool]

1. Rename %Maxwell{} to %Maxwell.Conn{};
2. Move helper functions to conn.ex;
3. Clean some badly code style.

PS: README is too long and hard to understand, maybe you should clear up when you free. :)
@secretworry

[coveralls]

Coverage decreased (-2.7%) to 95.29% when pulling 02ae14a on better-document into db1f3b9 on master.

[coveralls]

Coverage decreased (-2.7%) to 95.29% when pulling a8a7cd5 on better-document into db1f3b9 on master.

[coveralls]

Coverage decreased (-3.7%) to 94.306% when pulling 0c375dc on better-document into db1f3b9 on master.

[coveralls]

Coverage decreased (-3.7%) to 94.306% when pulling 4ffb463 on better-document into db1f3b9 on master.