support for prelude.rb

[kyrylo]

Description by @jfirebaugh: banister/pry-doc#1

[kyrylo]

Ruby-2.0.0-p0 supports Mutex#synchronize.

% pry
[1] pry(main)> pry-version
Pry version: 1.0.0.pre1 on Ruby 2.0.0.
[2] pry(main)> ? Mutex#synchronize

From: thread.c (C Method):
Owner: Mutex
Visibility: public
Signature: synchronize()
Number of lines: 2

Obtains a lock, runs the block, and releases the lock when the block
completes.  See the example under Mutex.

However, Thread.exclusive is still bugged.

[1] pry(main)> ? Thread.exclusive
Error: Could not load source for : No such file or directory - <internal:prelude>