Update to Lucene 4.9.0

Update to elasticsearch 1.3.0
Move to java 1.7

Related to #67.
Closed #76.
Deprecate `content` by `_content`

When we want to force some values, we need to set those using `_field` where `field` is the field name we want to force:

```
{
  "file": {
    "_name": "myfilename.txt"
  }
}
```

But to set the content itself, we use `content` field name.

```
{
  "file": {
    "content": "VGhpcyBpcyBhbiBlbGFzdGljc2VhcmNoIG1hcHBlciBhdHRhY2htZW50IHRlc3Qu",
    "_name": "myfilename.txt"
  }
}
```

For consistency, we set `_content` instead:

```
{
  "file": {
    "_content": "VGhpcyBpcyBhbiBlbGFzdGljc2VhcmNoIG1hcHBlciBhdHRhY2htZW50IHRlc3Qu",
    "_name": "myfilename.txt"
  }
}
```

Closes #73.
Log tika exceptions

Currently tika exceptions are swallowed with no log message.
We'd like to be able to know when/if this occurs and for what reason.

Closes #78.