Fix Regexp.escape to escape \t, \r, \n, \f characters
Add Regexp.quote as an alias of Regexp.escape
Cleanup Regexp.escape
Kernel#is_a? should check against included modules as well
Also catch Errno::EPIPE in uglify and gzip
Improve the Travis configuration

Now it should be less heavy on Travis and more useful, since the rspec
will be ran on all Ruby implementations but mspec will be ran only on
latest Ruby.
Alias Native::Array#to_ary to #to_a
Remove Native::Object#to_ary

Native::Array isn't really eligible for #to_ary status, so it ends up
raising weird exceptions when methods that use #to_ary work on it.
Add an Opal.respond_to? helper
Cleanup Array#first
Cleanup Array#last
Ensure Array#flatten doesn't explode with native objects
Ensure Array#join raises a comprehensible error with native objects
Cleanup Array#reject!
Fix Opal.respond_to? helper
Add Opal.inspect helper
Use Opal.inspect in Array#join
Properly coerce to String in String#end_with?
Properly coerce to String in String#start_with?
Fix String#end_with? in IE6..8

IE6..8 doesn't handle negative indices in String.prototype.substr