Debug mode

[sergeysova]

When debug is on:

• Add name and location to asserts (assertKeyframe, assertVariable, assertSelector)
• Asserts should show informative errors when used in incorrect way (ex.: keyframe used as a selector)
• Do not minify CSS (if not forced)

When debug is off:

• Do not add names (if not forced)
• Replace asserts with getters without informative errors, that should help end-users to report problem