Some minor updates to gemspec.
Some minor updates to gemspec.

Grab version from VERSION. This is already being updated, so let's make use of it.
Add website
Dynamically add executables

Seanstoppable/schema-evolution-manager,keithlayne/schema-evolution-manager,jonathang/schema-evolution-manager,mbryzek/schema-evolution-manager,mbryzek/schema-evolution-manager