Made Ethernet library non Blocking

Hi Guys,

I've extended the `Ethernet` library to make it possible to work with it in a non blocking way, which makes it easier to combine the library with handeling other task (handeling buttons, updating a screen etc) while a DHCP lease happens, or a packet is sent.

I've added non blocking variants for:

    - Dhcp::*
    - Dns::getHostbyName
    - Ethernet::begin (DHCP version)
    - Ethernet::maintain
    - EthernetClient::connect(*)

I welcome comments on the api-style, not really glad with all the names, but what would you guys think? I also need to think about a non blocking send, since it now loops until enough memory is available, I could make a variant which actually returns the bytes it could send, but I still have to think on the best way to do this.

There are 2 commits in this pull request, the Ethernet library was a mix of 3 different indentation styles, so I looked at the other parts and made commit a2ca62d to first only fix the formatting of the code. Then there is a new commit which actually changes the code, commit 2ac4e2f . This is actually an import from the work I started in DavyLandman/NBEthernet where I found out that the existing Ethernet library was already half non blocking.

So when reviewing, make sure to select just the correct commit (2ac4e2f), else in the github summary will claim I changed almost everything..

Impact on the size is quite minimal (for example ``DnsWebClient`` ): 
```
old: Binary sketch size: 13,474 bytes (of a 30,720 byte maximum)
new: Binary sketch size: 14,980 bytes (of a 30,720 byte maximum)
```

The increase is primarily due to keeping a blocking variant.