@Nullable results, initial

For now, only methods which may directly return null are annotated as @Nullable
delta in data
transitivity for @Nullable result inference
delta in data
for @Nullable result: resolving virtual call to this (sound)
delta in data
extracting Subroutine
Extended analyzer
@Nullable result: one more iteration

Changes:
1. Set of dereferenced calls during building a fix-point of frames
2. Only calls to `this` are de-virtualized
delta in data
just data for @Nullable result
an attempt to propagate nullity information

this reduces the number of false positives
delta in data
aggressive propagation of @NotNull information

it removes a lot of false positives
delta in data
Merge pull request #29 from ilya-klyuchnikov/nullable-result

inference of @Nullable results