Dev phrase cluster

**Goal:** reduce phrase index I/O by using the actual most significant term of each phrase for clustering rather than the 1st term.

**Result:** Great success.

Will merge shortly, updating indexes now.

    CARMEN_DIR=dev ./carmen.js --query="main st"
    Tokens
    ------
    main, st

    Result (showing 2 of 2)
    -----------------------
    - Main St x50
    - Stratford Rd 

    Stats
    -----
    - warmup:    1609ms
    - search:    76334 @ 125ms
    - relev:     26708 @ 381ms
    - results:   51 @ 63ms
    - relevance: 0
    - totaltime: 569ms
    Cache
    -----
    - degen:     4
    - freq:      0
    - term:      22
    - phrase:    957
    - grid:      102
    - total:     1085

    ------

    CARMEN_DIR=dev-phrase-cluster ./carmen.js --query="main st"
    Tokens
    ------
    main, st

    Result (showing 2 of 2)
    -----------------------
    - Main St x50
    - Stratford Rd 

    Stats
    -----
    - warmup:    562ms
    - search:    76399 @ 89ms
    - relev:     26734 @ 361ms
    - results:   51 @ 67ms
    - relevance: 0
    - totaltime: 518ms
    Cache
    -----
    - degen:     4
    - freq:      0
    - term:      22
    - phrase:    22
    - grid:      17
    - total:     65