README: Add info on debugging Gulp tasks

Close #448
README: Fix broken link.
Simplify the release process

Stop increasing the version in `package.json` after a release, do the
change just before the next release.

Add a shell script for making the release in one step.

Rename ChangeLog to ChangeLog.md, alter tools/git-changelog to produce
markdown.
ChangeLog: update to the new markdown format

Use the new style as introduced by 9c19bacc
HeapProfiler API

Implemented:
HeapProfiler.getProfileHeaders
HeapProfiler.startTrackingHeapObjects
HeapProfiler.stopTrackingHeapObjects
HeapProfiler.getHeapSnapshot
HeapProfiler.removeProfile
HeapProfiler.clearProfiles
HeapProfiler.takeHeapSnapshot
HeapProfiler.addProfileHeader
HeapProfiler.addHeapSnapshotChunk
HeapProfiler.finishHeapSnapshot
HeapProfiler.reportHeapSnapshotProgress
HeapProfiler.lastSeenObjectId
HeapProfiler.heapStatsUpdate
HeapProfiler.getObjectByHeapObjectId