bem version helper command

`bem version` helper command:
- shows current version of node, libs and current package when invoked with no args
- bumps version of current package in `package.json` and `bower.json` files (if exists), commits changes
- merges current branch when `--merge` or `--merge-into=branch` options are specified
- creates version tag when `--tag` option is specified

### Examples
#### Show current versions

```
$ bem version
{
  "http_parser": "1.0",
  "node": "0.8.25",
  "v8": "3.11.10.25",
  "ares": "1.7.5-DEV",
  "uv": "0.8",
  "zlib": "1.2.3",
  "openssl": "1.0.0f",
  "bem": "0.6.6"
}
```

#### Bump patch version

```
$ bem version patch
01:16:39.741 - info: Stage .json files
01:16:39.751 - info: Commit .json files
01:16:39.767 - info: Current branch is master
v0.0.5
```

#### Merge current release branch into `master`

```
$ bem version minor --merge
01:17:19.604 - info: Stage .json files
01:17:19.615 - info: Commit .json files
01:17:19.631 - info: Current branch is release/0.1.0
01:17:19.631 - info: Checkout branch master
01:17:19.640 - info: Merge release/0.1.0 into master
v0.1.0
```

#### Create version tag after commit

```
01:19:06.312 - info: Stage .json files
01:19:06.321 - info: Commit .json files
01:19:06.345 - info: Current branch is master
01:19:06.345 - info: Create tag v0.2.0 on branch master
v0.2.0
```

#### Use together with `git-flow`

```
$ git flow release start 0.2.0
Switched to a new branch 'release/0.2.0'

Summary of actions:
- A new branch 'release/0.2.0' was created, based on 'dev'
- You are now on branch 'release/0.2.0'

Follow-up actions:
- Bump the version number now!
- Start committing last-minute fixes in preparing your release
- When done, run:

     git flow release finish '0.2.0'

$ bem version 0.2.0
01:14:59.039 - info: Stage .json files
01:14:59.049 - info: Commit .json files
01:14:59.064 - info: Current branch is release/0.2.0
v0.2.0

$ git flow release finish 0.2.0

Switched to branch 'master'
Deleted branch release/0.2.0 (was 1f019de).

Summary of actions:
- Latest objects have been fetched from 'origin'
- Release branch has been merged into 'master'
- The release was tagged 'v0.2.0'
- Release branch has been back-merged into 'dev'
- Release branch 'release/0.2.0' has been deleted
```