Handle underscores in project and context names
Improve portability for Mac OS X

In particular, Mac OS X's off-the-shelf sed cannot handle
some regexps which are used by GNU sed.
Fix for next tasks in blocked projects

When a subtask of a blocked supertask was tagged with its own project, it
could end up as a next action in todo.txt.

For example, assume an empty todo.txt and outline.txt as follows:
Project A @someday +project
	Task A.1 +subproject

Then, "todo.txt ol sync" added Task A.1 to todo.txt
Merge pull request #2 from andreaswachowski/tweaks

Two small tweaks and a bugfix for outline-todo.txt