add Tor.is_ready and Tor.become_ready

[meejah]

See also #251 which this partially solves

[coveralls]

Coverage decreased (-0.2%) to 99.782% when pulling 077ba32 on is-dormant into 079a749 on master.

[codecov-io]

Codecov Report

Merging #252 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #252      +/-   ##
==========================================
+ Coverage   99.94%   99.94%   +<.01%     
==========================================
  Files          20       20              
  Lines        3663     3671       +8     
==========================================
+ Hits         3661     3669       +8     
  Misses          2        2

Impacted Files	Coverage Δ
txtorcon/controller.py	100% <100%> (ø)	⬆️

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[coveralls]

Coverage increased (+0.0001%) to 99.946% when pulling 0601710 on is-dormant into 079a749 on master.

[coveralls]

Coverage increased (+0.0001%) to 99.946% when pulling 92badf6 on is-dormant into 079a749 on master.

[coveralls]

Coverage increased (+0.0001%) to 99.946% when pulling 92badf6 on is-dormant into 079a749 on master.