Initialize Client *b to NULL

Issuing a focus up command when at the top of the client list caused
a crash due to Client *b not being initialized
Support ModX Bind.Mod values besides just Mod4

Allow user to specify Mod1, Mod2, etc. as Bind.Mod values
Merge pull request #50 from pfernie/focus_up

Initialize Client *b to NULL
Merge pull request #51 from pfernie/mod_mask_support

Support ModX Bind.Mod values besides just Mod4