Add new method WithTx for Database

[Xuyuanp]

Add a new method WithTx for Database, allowing use like

err := db.WithTx(func(tx *TxDatabase) error {
    // do something with tx
    return nil
})

[codecov]

Codecov Report

Merging #108 into master will decrease coverage by 0.04%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
- Coverage   86.35%   86.31%   -0.05%     
==========================================
  Files          38       38              
  Lines        2800     2805       +5     
==========================================
+ Hits         2418     2421       +3     
- Misses        330      331       +1     
- Partials       52       53       +1

Impacted Files	Coverage Δ
database.go	93.79% <60%> (-1.21%)	⬇️

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[codecov]

Codecov Report

Merging #108 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
+ Coverage   86.35%   86.45%   +0.09%     
==========================================
  Files          38       38              
  Lines        2800     2805       +5     
==========================================
+ Hits         2418     2425       +7     
+ Misses        330      329       -1     
+ Partials       52       51       -1

Impacted Files	Coverage Δ
database.go	96.55% <100%> (+1.55%)	⬆️

---

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[doug-martin]

@Xuyuanp thanks for the PR! If you could add a few more tests to cover your changes I'll get this into the next release.

[Xuyuanp]

it seems better this time