ProcessManager: check status() before reading or writing.
Merge pull request #72 from KamunagiChiduru/feature/check-status

ProcessManager: check status() before reading or writing.
ProcessManager: refactoring: separate internal status logic (refs #72)
ProcessManager: oops (refs #72)
ProcessManager: avoid duplicate computation (refs #72)
Revert "ProcessManager: avoid duplicate computation (refs #72)"

This reverts commit 62ba29ab8c9e6f5cdf42a22a9c7e84db5b3a0af1.
ProcessManager: alternative way (refs #72)
ProcessManager: check not only by signal 0 but also by waitpit + pid change
ProcessManager: uses pgroup_open instead of popen3
Add doc of {limit} argument for flatten(). Closes #67
ProcessManager: more documented
ProcessManager: internal variable name refactoring
Process: refactoring: assigned, and was never used from anybody.
Process: refactoring: be more readable code; use less technique with preserving previous succinctness.
Process: add TODO
ProcessManager: use vimproc's checkpid instead of kill(0) to avoid zombies.
Database.Sqlite: initial prototype
Database.Sqlite: more documented
Database.Sqlite: query() also parse/construct result
Database.Sqlite: more principled